What does “hydrostatic head” mean, and why does it matter?

Hydrostatic head measures how much water pressure a fabric can withstand – essentially, it tells you how waterproof the material is.

Imagine a vertical tube with a fabric sample sealed at the base – now add water to the tube. The height of the water column (in millimeters) at the moment water pushes through the fabric/coating is its hydrostatic head rating.

There’s no universal standard for how high this should be to consider as “waterproof”. This measurement is crucial when comparing dry bags and tents/shelters, where real-world performance depends heavily on water resistance.

Dry Bags

• A dry bag used inside a backpack will only require a modest hydrostatic head.
• Our Ultra-Sil Dry Bags are our go-to for storing gear inside your backpack. The fabric tests at over 2000mm (more waterproof than some tent floors), which is perfectly adequate for waterproof storage inside a pack, but not sufficient for use in paddle situations.
• A dry bag for paddling or water sports needs a much higher rating – at least 10,000mm, such as our Lightweight Dry Bags or Big River Dry Bags.
  
  

Tents and Shelters

• For moderate rain, a tent floor should have a hydrostatic head of 1200–2000mm, with a rainfly around 1200–1500mm.

• For torrential rain or boggy ground, look for a floor rated 4000–5000mm or higher, and a rainfly ideally over 1500mm.
  
  

If a manufacturer doesn’t publish the hydrostatic head for a dry bag or tent, it’s worth choosing a product that does.

Read more in our blog post: Just How Waterproof is Waterproof?